Recognition of Hepatitis B Antigen Determinants by a Double Label Radioimmunoassay: A Sensitive Means of Subtyping Hepatitis B Antigen

A sensitive radioimmunoassay is described for subtyping hepatitis B antigen (HB Ag) in human serum. The method utilizes antisera (HB Ab), prepared in immunized guinea pigs, which contain predominantly anti-d or anti-y antibodies, and radioiodinated HB Ag of either D or Y subtype. Sera with HB Ag of subtype D (D) were strongly positive by RIA, employing anti-d HB Ab and 125I-HB Ág (D), whereas specimens with HB Ag of Y subtype were negative or only weakly positive. Converse results were observed when antigenemic sera were evaluated by RIA with anti-y HB Ab and 125I Y. By combining guinea pig HB Ab possessing high-titered anti-d and anti-y with 131I D and 125I Y, it was possible to screen for and subtype HB Ag simultaneously. One hundred and five antigenemic sera were evaluated; 93 or 88% could be readily subtyped by the combined RIA, including 31 of 43 samples with HB Ag in amounts too low to be detected by agar gel diffusion (AGD) or counter immunoelectrophoresis (CEP). Subtype D-HB Ag was found primarily in the serum of blood donors and patients with posttransfusion hepatitis, whereas subtype Y antigen predominated in individuals whose hepatitis was associated with the illicit use of drugs. Samples showing only “a” specificity by AGD also possessed “d” or “y” determinants by RIA. The relative antigenic reactivity of “a” and “d,” or “a” and “y” determinants appeared to vary in different specimens. This could not be explained simply by the HB Ag subtype or the number of particles in the specimens evaluated.